Silicon accelerometers have a wide variety of applications because of small size, low mass and low cost. The uses of accelerometers, in particular micro silicon based accelerometers, are many and varied and new applications will certainly be found as the manufacturing technology develops. In particular, the automotive industry is one of the biggest users of these devices. The increasing use of electronics systems in automobiles to replace existing systems based on mechanical devices will see substantial use of silicon based micro accelerometers in the automotive industry. At the present time, silicon based micro accelerometers are being used in suspension systems, braking systems, traction control systems, steering systems and safety systems.
Silicon accelerometers in their most useful configuration at present have two different forms. One form is a low resonant accelerometer having a resonator. In this form, two vibrating beams each support on one end respective masses. The two beams longitudinally lie on the same axis. Acceleration of the device causes opposite perturbations in the resonant frequencies of the beams, because inertial forces on the respective masses compress one beam and stretch the other. The output signal of the device is the frequency difference between the two beams. The resonator can be driven by electrical forces, thermal excitation, laser excitation or the like.
The other form is a piezoresistive accelerometer. In this form, a mass is suspended by two beams attached to opposite sides of the mass. When the system is accelerated, the beam-mounted mass will, on account of its inertia, at first lag behind the movement, one beam will thus be stretched and the other will be compressed which cause changes in the beams. The force acting on each beam is equal in magnitude and is equal to the product of the weight of the mass and the acceleration experienced by the mass. The output signal of the device is the resistance difference between the two beams.
Both kinds of accelerometers have silicon beams which are supported by a silicon substrate. Forming silicon beams is the key step in the fabrication of these accelerometers. A common method is to etch a hole upward from the bottom of a substrate, leaving thin silicon beams suspended over the hole. The whole structure is then eventually bonded to a base.
FIG. 1 shows a conventional silicon beam piezoresistive accelerometer. This accelerometer comprises a substrate 10, two beams 12 each of which has a P-type region 28 on the surface, a mass 16, a cavity 18, a base plate 46 and a cap 48. When the accelerometer is fabricated, the back side of the substrate is anisotropically etched using an etchant to form the beams and the cavity and the upper side is processed to form the resistors 28 and all other device elements. The double sided processing method has several problems to be solved. Among them are: a. Processing steps are very complicated because proper alignment of work done on both-side is required and difficult to achieve; b. etching of 400 microns thick silicon results in beam thickness variations of more then 3 microns which introduce substantial errors and create other problems; c. the dimension of the beam can not be made small because etching in the lateral direction results in poor dimension control.
These problems are obstacles to enhancement of measuring accuracy, minuaturization, mass production and reduction in cost.